The continued advances in high power switching semiconductors and control electronics have enabled switched reluctance machines, which have been used extensively in motor applications with great success in the past, to be exploited increasingly for the generation of electric power. Such use is highly desirable in light of the simple and rugged winding-free, magnet-free, brushless construction of the salient pole rotor. This rugged construction allows the machine to be run at high speeds and in very harsh environmental conditions. Additionally, since the rotor does not have windings or magnets, its cost is less than a wound or permanent magnet rotor.
A typical commutation approach which is used to allow this generation of electric power with a switched reluctance machine is illustrated by the simplified schematics of FIGS. 1a and 1b viewed in conjunction with the graphs of FIG. 2. As the rotor rotates, the inductance of the stator winding varies as the salient rotor poles come in and out of alignment with the stator poles, as illustrated by trace 20 of FIG. 2. To allow electric power generation, the switches 22, 24 (typically electronic semiconductor devices) are closed to allow current to flow from the bus 26 to energize the stator winding 28 as indicated by arrows 30, 32. This turn-on occurs at a turn-on angle after alignment of the rotor and stator pole has begun, as indicated as axis 34 on FIG. 2 (indicated as TURN.sub.-- ON). Once both switches 22, 24 are closed, the current through the stator winding 28 increases as indicated by trace 36 on FIG. 2. At axis 38, the rotor pole and the stator pole are aligned and the inductance peaks. After this point the inductance begins to decrease, which results in a rapidly increasing current. Once this increasing current exceeds the upper current limit (I.sub.HI) at axis 44, both switches 22, 24 are opened (see FIG. 1b) and the current is returned to the bus 26 through diodes 40, 42. Due to the decreasing inductance during this phase, the current delivered to the bus continues to increase. Once, however, the rotor and stator pole are unaligned beginning at axis 46, the inductance has reached its minimum value (at which it remains until the rotor and stator poles begin to come into alignment again). During this period, the current delivered to the bus decays until it reaches zero. Because more current is generated during the decrease in inductance as the rotor pole pulls away from the stator pole than is required to be supplied by the bus, a net generation of electric power occurs.
The voltage response of such an electric power system including a switched reluctance generator is dominated by the characteristics of the voltage loop control. This is a result of the differences between a synchronous generator, which is typically used, and a switched reluctance generator. This difference arises because a synchronous generator is essentially a voltage source with an internal reactance that causes the output voltage to decrease as the load increases. With no changes in the excitation current, an electrical system with a synchronous generator will maintain a relatively constant voltage for a wide range of loads. A switched reluctance generator, on the contrary, is essentially a controlled current source that requires continued controller action to produce the desired voltage. With no changes in the commands to the inverter of a switched reluctance generator, the bus voltage either increases significantly or decreases to zero with only moderate changes in load. This is a significant problem which heretofore has impeded the utilization of the switched reluctance machine in regulated voltage power generation systems.
In order for a switched reluctance generator to provide regulated voltage it is necessary to close a voltage control loop. The conventional approach for obtaining a high bandwidth voltage control loop is to develop a linear relationship between the command DC link current and the actual DC link current. This linear relationship is developed, in prior systems, by repeatedly running an analysis program and obtaining a map of the control variables, operating point, and DC link current. For a switched reluctance generator, the control variables are the turn-on angle, turn-off angle, and the commanded currents, and the operating conditions are generator speed and bus voltage. Once a map of control variables versus desired DC link current is determined for a particular machine, it is implemented in the controller and utilized to control the generator operation. Since this approach relies on a map of the machine performance characteristics (analytical or measured) of a particular machine or small group of machines, differences between analytical and actual characteristics and variations between specific generators can have a significant impact on the performance of any particular generator. One such parameter which varies from one machine to the next and which has been shown to cause significant differences in the switched reluctance generator performance characteristics is the airgap thickness. Since it is difficult and burdensome to maintain a specific controller with a specific machine to overcome this problem, the mapping approach does not provide the robust linearization necessary for high performance electric power systems.
The instant invention is directed at overcoming one or more of the above identified problems existing with the prior art.